The present disclosure relates generally to aircraft and, in particular, to aircraft structures. Still more particularly, the present disclosure relates to stringers and other structural designs for an aircraft.
Aircraft are being designed and manufactured with increasing percentages of composite materials. Some aircraft may have more than 50 percent of their primary structures made from composite materials. Composite materials may be used in aircraft to decrease the weight of the aircraft. This decreased weight may improve payload capacities and fuel efficiencies. Further, composite materials may provide longer service life for various components in an aircraft.
Composite materials are typically tough, lightweight materials created by combining two or more dissimilar components. For example, a composite material may include fibers and resins. The fibers and resins may be combined to form a cured composite material.
Further, by using composite materials, portions of an aircraft may be created in larger pieces or sections. For example, a fuselage in an aircraft may be created in cylindrical sections that may be put together to form the fuselage of the aircraft. Other examples may include, without limitation, wing sections joined to form a wing or stabilizer sections joined to form a stabilizer.
A stringer is an example of a component that may be manufactured from composite materials. A stringer is an elongate member and is configured for attachment to another structure, such as a panel. For example, a stringer may be attached to a skin panel for an aircraft. This skin panel may be used in a wing, fuselage, or other component in the aircraft. The stringer also may help carry and/or transfer loads. For example, a stringer may transfer a load from a skin panel to another structure. This other structure may be, for example, a frame or a rib.
Composite skin-stringer interaction structures are susceptible to delamination under dominant loads. The problem of skin-stringer interface cracking (delamination) is detrimental to the integrity of an aircraft structure such as a wing. Traditional design rules (balance or symmetry) in composite layups are used to avoid undesired failure modes and coupling effects, but these design rules restrict the stacking sequences.
Existing solutions include increasing the thickness of stringer and/or skin at a trouble spot; and redesigning the skin and stringers next to the problem area to diverge the load from the trouble spot. These solutions lead to weight penalty, manufacturing costs, and additional time spent on redesign.
It would be advantageous to provide skin-stringer structures that are less susceptible to interface cracking.